Biocomposites from wheat proteins and fibers: Structure/mechanical properties relationships

For the first time, biocomposites from wheat by-products, i.e. wheat gluten and wheat straw fibers, were prepared using a thermomechanical process. Three types of wheat straw fibers displaying contrasted sizes (median diameter d50 = 1.1 mm, 62 μm and 8 μm), morphologies and surface reactivities were prepared by successive grinding processes (cut milling (CM), impact milling (IM) and ball milling (BM), respectively). The main objectives of the present study were (i) to gain further scientific knowledge about the impact of fibers characteristics on the mechanical properties of these composite materials, and (ii) to evaluate in which extent the addition of wheat straw fibers could decrease the final cost of materials without altering their mechanical properties. Mechanical properties were assessed through tensile tests and discussed in relation to the structure of materials, especially to the interfacial adhesion between the fiber and the matrix. Globally, an increasing fiber content up to 11.1 vol% led to an increase in both the Young's modulus and the stress at break, and to a reduction in the strain at break. Experimental Young's moduli were well predicted by the Tsai–Pagano model for the three types of fibers. Data of specific mechanical energy consumed upon processing, contact angle measurements and observations of cryo-fractured surfaces allowed to demonstrate that the adhesion at the interface wheat straw fiber/wheat gluten was improved by (i) an increased hydrophobicity of wheat straw fibers, which was induced by successive grinding, and (ii) a higher specific surface of fibers which was favoured in the case of small fibers. The interfacial adhesion was improved in the order BM > IM > CM whereas the fiber aspect ratio decreased in the order CM > IM > BM. Thus, a competition between these parameters could occur, explaining why no significant difference in reinforcing effect was achieved between the three types of fibers. Finally, the reinforcing effect of wheat straw fibers was also ascribed to a deplasticizing effect, which was particularly pronounced for BM wheat straw fibers.

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► Biocomposites from wheat by-products were prepared using solvent free processes.
► Three types of wheat straw fibers (median diameter d50 from 18 to 1100 μm) were obtained by successive dry grinding.
► Successive grinding induced an increase of the hydrophobicity of wheat straw fibers.
► An increased hydrophobicity of fibers favoured the adhesion at the interface fiber/matrix.
► The use of wheat straw fibers allowed reducing the final cost of biocomposites.